The simplest technical solution to meet standards on NOX emissions for marine vessels is also the one that minimizes total cost of ownership and maximizes uptime.

Both Selective Catalytic Reduction (SCR) and Exhaust Gas Recirculation (EGR) technologies enable Cat® marine engines to meet Tier 4 Final and IMO Tier III emissions standards — but SCR also provides measurable fuel cost savings, uses a proven engine platform, and offers simple installation and maintenance. That’s why Caterpillar chose to implement SCR across its Tier 4 Final marine engine product line.

“The decision came after careful evaluation of the technical options available and a comparison of each of these options against critical customer requirements in the marine industry,” says Jason Spear, Caterpillar Marine Product Development Manager said. “Our engineers were able to place their focus on driving down fuel consumption instead of adapting a technology unproven in the marine marketplace.”

Reduced Fuel Consumption

SCR technology allows for advances in combustion and fuel efficiency through NOX reduction downstream of the engine itself. The Caterpillar SCR solution involves injecting Diesel Exhaust Fluid (DEF) into the exhaust system. There, it decomposes into NH3, which selectively reduces NOX over a catalyst. Many customers expect that if emissions decrease, fuel consumption will increase. But that’s not the case here.

“The relationship between engine-out NOX and fuel consumption is inversely proportional,” Jason Spear says. “This means SCR maximizes engine efficiency. The amount of diesel fuel required to run a Tier 4 Final engine from Caterpillar with SCR can be up to 9 percent lower than that required for a Tier 2 or Tier 3.”

This savings in diesel fuel more than offsets the cost of DEF, enabling the SCR solution to deliver the lowest total cost of ownership in the marine sector.

Simple, Durable Design

Just as important, the Tier 4 Final marine engine solution from Caterpillar is durable, reliable and technically robust — due in large part to its simplicity. EGR solutions, on the other hand, require complex interactions of mechanical, electrical and fluid processes working together in perfect unison.

“The SCR option is not about redesigning the engine,” Spear says. “Meeting Tier 4 Final using solely EGR has implications for engine efficiency and durability. Dramatic technological countermeasures can soften the impacts, but not eliminate them.”

Caterpillar addressed any technical challenges arising from SCR with straightforward, practical solutions. Expectations of variable sulphur content in fuel, for example, led the company to design a more sulphur-tolerant SCR solution, allowing for operation in regions with higher sulphur content fuels.

Easy Installation and Maintenance

The Caterpillar SCR solution is also simple to install — minimizing design and installation costs. The company developed a variety of different orientations for housing the additional equipment, depending on the ship installation and whether the kit is destined for the stack or the engine room.

“The feedback we got from naval architects was to make it as flexible to install as possible,” Spear says. “That has been our guiding principle.” 

It’s also fully accessible for service engineers, reducing downtime and replacement/overhaul parts costs. Unlike EGR options, there is no engine-related service such as stricter fuel filtration requirements or EGR valve and cooler maintenance. SCR service periods generally align with engine overhaul intervals.

Proven Industry Solution

Simplicity and reliability mean SCR has already earned a place in the marine industry. Over the past decade, more than 1,250 SCR systems have been installed on marine vessels, including on-board ferries, tankers, container ships, icebreakers, cargo ships, workboats, cruise ships and naval vessels.

By comparison, EGR is relatively unproven in marine operations long term. To meet marine emissions regulations, it re-circulates a portion of exhaust gas to the engine cylinders, reducing oxygen content intake. This lowers combustion temperatures and generates less NOX, but it’s also less efficient from a combustion standpoint. Marine EGR proponents are addressing that shortcoming through the use of common-rail direct fuel injection systems operating at higher pressures.

However, Name sees the resulting increases in preventive maintenance and service activities — such as the need to exchange fuel filters more frequently — as problematic for customers.

“The EGR option cannot claim validation in the harsh environments that engines face over the traditional useful life expectations operators demand,” he says. “We’re very comfortable that we have not developed our Tier 4 Final marine solution around a technology that’s untried in its market, which could compromise customer uptime."